
function fn(arr1,arr2){
var   arr = arr1;
var   x = 1;
  if(arr1.length === arr2.length){
    for(i=1;i <= arr2.length;i++){

    arr.splice(x,0,arr2[i-1]);
    x = x + 2;
    // console.log(arr);
  }
}else{
  console.log("I do not know");
 }

 console.log(arr);
}

fn([1,2,3],[4,5,6]);
